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Page 71 Listening to Light — Teacher Notes 5.1 WHAT'S THIS ABOUT? Students learn that light carries information and that infrared (IR) radiation is a form of light that in some cases behaves like visible light and other cases behaves very differently. Students first see how a photocell (solar cell) can be used to detect the presence of light. They then learn how the photocell reacts to light from a laser pointer (or other laser) and a remote control, and see that information, for example a message, can be transmitted by visible and infrared light. They listen as an infrared-emitting diode is used to transmit music from an audio source (like a CD player) to the photocell with a speaker connected to it. Finally, students test the effects on the transmission of music when various objects are placed in between the infrared- emitting diode and the photocell. They learn that some objects that block visible light allow infrared light to pass through. Grade Levels 7-12 Time Required 50-100 minutes, 1 to 2 class periods Learning Objectives After completing this activity, students will be able to: Describe different characteristics of visible and infrared light observed using a photocell as a detector. Explain how information can be carried by visible and infrared light. Predict whether visible or infrared light will be transmitted through various objects. Student Prerequisites and Misconceptions Students are assumed to be familiar with visible light and infrared light, and that both are part of the electromagnetic spectrum. For an introduction to the electromagnetic spectrum, see: http://imagers.gsfc.nasa.gov/ems/ems.html This activity assumes some familiarity assembling and working with electronic circuits, magnifying glass lenses, laser light, and remote controls. Common misconceptions are described in the overview for these activities, "Active Astronomy : Classroom Activities for Learning about Infrared Light,” page 3.

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Page 1: Listening to Light — Teacher Notes · Listening to Light — Teacher Notes ... When visible or infrared light strikes the solar cell, ... (next to the plastic piece

Page 71

Listening to Light — Teacher Notes

5.1 WHAT'S THIS ABOUT?

Students learn that light carries information and that infrared (IR) radiation is a form oflight that in some cases behaves like visible light and other cases behaves very differently.Students first see how a photocell (solar cell) can be used to detect the presence of light. Theythen learn how the photocell reacts to light from a laser pointer (or other laser) and a remotecontrol, and see that information, for example a message, can be transmitted by visible andinfrared light. They listen as an infrared-emitting diode is used to transmit music from an audiosource (like a CD player) to the photocell with a speaker connected to it. Finally, students testthe effects on the transmission of music when various objects are placed in between the infrared-emitting diode and the photocell. They learn that some objects that block visible light allowinfrared light to pass through.

Grade Levels7-12

Time Required50-100 minutes, 1 to 2 class periods

Learning ObjectivesAfter completing this activity, students will be able to:• Describe different characteristics of visible and infrared light observed using a photocell as a

detector.• Explain how information can be carried by visible and infrared light.• Predict whether visible or infrared light will be transmitted through various objects.

Student Prerequisites and Misconceptions• Students are assumed to be familiar with visible light and infrared light, and that both are part

of the electromagnetic spectrum. For an introduction to the electromagnetic spectrum, see:http://imagers.gsfc.nasa.gov/ems/ems.html

• This activity assumes some familiarity assembling and working with electronic circuits,magnifying glass lenses, laser light, and remote controls.

• Common misconceptions are described in the overview for these activities, "ActiveAstronomy : Classroom Activities for Learning about Infrared Light,” page 3.

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SOFIA SCIENCELong light waves ranging from03 to 1600 micrometers arenormally invisible to humaneyes. SOFIA’s electronicdetectors and computers willcovert light gathered by thetelescope into signals thathumans can understand.

BACKGROUND INFORMATION ON THE SCIENCE & TECHNOLOGY

In this lecture demonstration, an infrared light-emitting diode (LED) is used to transmit musicacross open space to a speaker. The music can come from a Walkman, stereo, CD player,computer, amplified microphone, or any similar device. The transmitter circuit contains aninfrared Light Emitting Diode (LED) whose brightness varies in response to changes in inputcurrent. The receiver circuit uses a photocell to detect the IR signal and convert it back to anelectrical signal for the speaker. The student activity sheets call the receiver circuit the“photocell detector.”

After showing the basic operation of this demonstration, theequipment can be used to investigate some basic properties ofthe infrared light coming from the diode. First, and mostobvious, is that the IR light is invisible. Second, a magnifyingglass can be used to show that infrared signal is focused thesame as visible light. (Note: Glass is not transparent to allinfrared light. It is transparent to near-infraredlight—wavelengths close to visible light. The LED used in thisactivity emits near-infrared light. Glass is opaque, however, toinfrared light with longer wavelengths.) Finally, differentmaterials are placed between the diode and the photocell toshow that the IR light can pass through many materials that visible light cannot.

Light-Emitting Diodes (LEDs) can be thought of as tiny light bulbs. (Note: A diode is a siliconcrystal (semiconductor) that allows current to flow in one direction only, kind of like a one-wayturnstile for electrons.) When a current is applied to an LED, electrons in the semiconductormaterial emit light. Because of the composition of the material in the LED used in this activity(usually a silicon compound), it emits infrared light, not visible light. The higher the incomingcurrent, the more light the LED will emit. If the electrical signal coming into the LED is notstrong enough, however, it will not emit any light. The battery and capacitor in the transmittercircuit provide a constant source of current that keeps the LED above this threshold, so it emits atleast a constant amount of light, regardless of the incoming audio signal. Since the receivercircuit does not produce any sound when exposed to a constant light source, the constant lightfrom the LED caused by the battery and capacitor is not detected. Instead, the receiver circuitdetects changes in the brightness of light emitted by the LED, changes that are caused byfluctuations in the incoming current from the audio source that are added on top of the battery’sconstant one.

Solar/Photo Cells: The solar cell used in this activity is known as a photovoltaic cell because itconverts light (photo) into electricity (voltaic). When visible or infrared light strikes the solarcell, some of it is absorbed by the special material (usually silicon), called a semiconductor, outof which it is made. The energy of the absorbed light is transferred to the semiconductormaterial, knocking electrons in it loose. These loose electrons can then move freely throughoutthe material, resulting in an electric current. Most solar cells also have one or more electric fieldsin them which force the moving electrons to flow in one direction. By placing metal contacts atthe top and bottom of the solar cell, the current flowing within it can be drawn off for use

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externally. In the case of this activity, the current generated by the light is sent to theamplifier/speaker, where it is converted into sound waves, which we can hear. For more detailedinformation about how a solar cell works, see: http://www.howstuffworks.com/solar-cell.htm

Audio Speakers: Inside a speaker, a flexible cone (usually made of paper, plastic, or metal)vibrates rapidly in response to a changing electrical current. As it moves, it pushes the airmolecules around it. Those air molecules, in turn, push the air molecules near them, and thevibration is transmitted through the air as a sound wave. Our ears detect the vibration of airmolecules and convert them into an electrical signal that our brain interprets as music. Within thespeaker, the flexible cone needs a changing current to vibrate. If the current is constant, the conewill not vibrate. For more information on how speakers work, see:http://www.howstuffworks.com/speaker.htm

Remote Controls: When you press the button of a remote control, an electrical connection ismade that tells a computer chip inside the remote which button was pressed. The chip thenproduces a morse-code-like electrical signal that is different and distinct for each button.Transistors inside the remote control amplify the signal and send it to a Light-Emitting Diode, orLED, a kind of small light bulb. The LED converts the electrical signal into infrared light.Because the LED emits infrared light, which our eyes cannot detect, we do not see any lightpassing between the remote control and the TV (or VCR). But, the TV (or VCR) has a detectorwhich can see infrared light. Depending on the exact nature of the signal (its wavelength,frequency, or intensity), the TV (or VCR) carries out the desired command. Note that manycamcorders can also detect infrared light. If you aim a remote control at a camcorder and pusha button, you should see infrared light flashing in the viewfinder. For more information abouthow remote controls work, see: http://www.howthingswork.com/inside-rc.htm

BUILDING THE DEMONSTRATION APARATUS

This lesson is described in the demonstration format based on the assumption that you will haveonly one apparatus available. Students in small groups can also build this apparatus if sufficienthardware is purchased.

The Receiver Circuit: How It Works

In this circuit, the photocell receives the IR signal from the LED and converts it to an electricalsignal that is sent to the amplifier-speaker. This circuit is also used in another activity, "Sensingthe Invisible."

PhotocellAmp/Speaker

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The photocell (or solar cell), connected as shown above, produces an electric current whenexposed to light. Because of the way speakers are constructed, a changing current is needed toproduce a sound in the speaker; a constant current will not produce a sound. When a constantlight source illuminates the photocell, it produces a constant current and no sound is produced.Students should hear static, if anything, when a constant light source illuminates the photocell.When the light changes in brightness, the current produced by the photocell also changesaccordingly, and the speaker will produce a sound. If the light is turned on and off (as happens ifyou move your hand back and forth in the beam of light), you will hear series of “pops” eachtime the light is turned back on. If the light varies because of a changing electrical current froman audio source, you will hear music from the speaker.

NOTE: For best results, turn off any overhead fluorescent lights in the classroom. When thephotocell is exposed to the fluorescent overhead lights, the speaker will emit a constant buzzingsound or hum. This buzzing occurs because the intensity of the classroom light fluctuates due tothe 60 Hz AC current it receives. Thus, fluorescent lights will generate a constant hum, whichmay interfere with students’ hearing sound produced from the flashlight and the infrared diode inthe transmitter circuit.

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How to Build the Receiver Circuit:

To make the photocell detector, use jumper cables to connect the photocell with theamplifier/speaker (amplifier/speaker requires a 9V battery). Clip one alligator clip from a jumpercable to one of the leads from the photocell, and clip the alligator clip at the other end of thejumper cable to one of the leads of the audio cable (which has its 1/8” mini-plug plugged into the“input” of the amp/speaker). Use a second jumper cable to connect the other lead from thephotocell to the other lead of the audio cable.

This works best if you use an audio cable with a mini-plug on one end, and two exposed wireleads on the other. You can also use an audio cable with a mini-plug on both ends. In this case,connect the alligator clip from one jumper cable near the end of the mini-plug, and connect thealligator clip from the other jumper cable near the base of the mini-plug (next to the plastic piecethat protects the rest of the audio cable). Be sure both alligator clips are in contact with metalparts of the mini-plug, and that they do not touch one another. Note that a Y-Adaptor AudioCable will not work in this activity.

How to Build the The Transmitter Circuit

This circuit takes the output audio signal from an audio source (CD player, transistorradio, stereo, etc.), in the form of a changing electrical current, and sends it to the infrared light-emitting diode (LED). The LED converts the current into infrared light, in much the same way aregular light bulb “shines” when the light switch is turned on.

Capacitor

LED Input signal

AA battery

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To make the transmitter circuit, use jumper cables to connect the LED to the battery, capacitor,and to the audio cable coming out of the audio source (CD player, transistor radio, etc.), asshown in the diagram above. Be sure to connect the audio cable to the Line Out jack of the audiosource (like a Walkman), (or the Headphone jack if the Line Out is note effective). Commentsabout connecting the jumper cables to the audio cable made in the section on the ReceiverCircuit, also apply here.

NOTE: YOU WILL NOT BE ABLE TO “SEE” WITH YOUR EYES IF THE LED ISEMITTING INFRARED LIGHT. Test the LED with the receiver circuit. If the LED does notwork (you get no sound from the speaker when the LED is placed just above the photocell),switch the polarity of the battery (i. e. take the jumper cable connected to the positive terminaland connect it to the negative terminal, and vice versa).

TEACHING NOTES ON "LISTENING TO LIGHT"

Part I - The Photocell: Students will use a detector to hear the presence of light.

These demonstrations show students how the photocell detector can be used to “listen” to light.For the first demonstration, turn off the overhead fluorescent lights. Use a DC battery poweredflashlight to shine on the surface of the photocell. Because the flashlight operates on DC current,

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there will be no fluctuations in the brightness of the light emitted and the speaker will be silent.The speaker only responds to changes in the voltage it receives, and a constant light source willcause the photocell to produce a constant voltage. However, if the teacher moves her or his handback and forth to block the beam reaching the photocell, the speaker will emit a sound. Thestudents should answer questions A-D while the teacher demonstrates the effects of the flashlighton the photocell detector. Question E should be answered before the teacher tests the students’predictions. Questions D and F allow the students to demonstrate an understanding that thephotocell detector enables them to listen to changes in the brightness of light, not how bright thelight is.

Part II - Your Own Music Station: Students discover that information can be carried by visibleand infrared light.

These demonstrations show how information can be carried by both visible and infrared light.The first demonstration involves a laser beam aimed at the photocell surface from across theroom.

SAFETY PRECAUTION: Be sure to caution students not to look into the laser beam.

Visible Light Demonstration

The laser should be aimed at the photocell surface before students answer Question A. Becausethe brightness of the laser light does not fluctuate, the speaker will emit no sound (although itmay make a popping sound when the laser pointer is initially turned on, followed by no sound, orstatic, as the laser pointer continues to shine on it). It is up to the students to suggest thatinterrupting the light signal will produce a sound, and that this method might be used to sendinformation along the laser beam (perhaps using Morse code). After the students have provideda method of communicating with their friend, the teacher can test their suggestions using theequipment.

Infrared Light Demonstration

When the teacher hits the photocell detector with the infrared signal from a remote control, thespeaker makes a sound like a blaster gun from a science fiction movie. Different buttons on theremote produce signals with different fluctuations, but these changes are difficult to hear with thesimple circuit used in the demonstration. If different brands of remote controls or differentremote control devices are used, the sounds are easier to distinguish. Question B should beanswered after the teacher demonstrates the response of the receiver to the remote control signal.

Before students answer Questions C, the teacher should demonstrate how the two circuits shownabove are used to transmit and receive music via an IR signal. For best results, aim the top (notthe side) of the LED directly at the photocell (so that the prongs on the LED point directly awayfrom the photocell). To begin, hold the LED close to the photocell (within an inch or two). Thisensures that students hear the music as loudly as possible at the beginning of the demonstration.Note that the music coming from the speaker will probably be rather quiet, even at its loudest.But it should still be easily heard, even if not loud.

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The volume of the music will go down if the LED is moved farther away from the photocellsurface. A simplified but appropriate explanation for this is that the light from the LED spreadsout as it moves farther away. As a result of this spreading out, it appears “dimmer” the fartheraway it is. However, this is not the whole story. The sound produced in the speaker does notdepend directly on the brightness of the light detected by the photocell, but rather on the changesin the brightness. As the LED moves farther away, and its light gets dimmer, the size of thebrightness fluctuations detected by the photocell necessarily decreases. The smaller fluctuationsin the light detected by the photocell result in smaller changes in the current sent to the speaker,and the resulting volume of the speaker is quieter.

Part III - What Gets Through? Students investigate the infrared transmission properties ofvarious everyday objects.

The first demonstration uses a magnifying glass to focus the infrared light from the transmitteron the distant photocell. The LED and photocell should be placed about 1 meter apart with theLED still aimed toward the photocell. The signal will be so weak that no sound is heard.However, when a magnifying glass is placed between the LED and the photocell the soundbecomes audible. The magnifying glass needs to be about 15 cm from the diode and should bemoved back and forth to find the optimal location.

If students are not familiar with how a magnifying glass focuses visible light, this can bedemonstrated using the flashlight. The fact that both visible light and infrared light are focusedby the magnifying glass provides evidence that IR light does, in this instance, behave very muchlike visible light. Note: Test the magnifying glass used prior to class. Different types of plasticand glass absorb IR emission, and you may find that your magnifying glass absorbs most of theIR from the LED.

The activities associated with Question C & D demonstrate that many materials that are opaqueto visible light are either totally or partially transparent to IR light.

Question E uses the observations from part III to help understand why astronomers are interestedin using telescopes that view infrared light. Infrared light can indeed carry information in muchthe way that visible light can, but it can pass through interstellar dust, which is opaque to visiblelight. If you would like to show students images of astronomical objects taken in differentwavelength ranges, see: http://coolcosmos.ipac.caltech.edu/cosmic_classroom/multiwavelength_astronomy/multiwavelength_astronomy/ (“Objects at Multiple Wavelengths”are also included in section 5 of the “Image File” on the CD-ROM).Note that objects at different temperatures emit light in different wavelength regions, with x-raysand gamma rays emitted by the hottest objects (millions and billions of degrees), and radiowaves emitted by the coolest (tens of degrees above absolute zero). For example, an x-ray imageof the Sun reveals gas in the outermost layers of the Sun at temperatures of millions of degrees,while a visible light image shows gas at much cooler temperatures (thousands of degrees) locateddeeper in the Sun.

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MATERIALS NEEDED

NOTE: One set of these materials is required to teach this activity as a demonstration. Or, ifsufficient materials are available, student can conduct the activities as a laboratory experiment inwhich case each group of students would require a set of materials.

Transmitter Circuit• Infrared Light Emitting Diode (LED)*• 0.22 µF (microfarad) Capacitor*• Audio Cable with 1/8” mini-plug on one end*• 5 Jumper Cables with alligator clips on both ends *• AA Battery• AA battery holder*

Receiver Circuit (Also used in "Sensing the Invisible")• Solar cell*• Amplifier/Speaker*• Audio Cable with 1/8” mini-plug on one end*• 2 Jumper Cables with alligator clips on both ends*• 9 Volt Battery for Amplifier/Speaker

Other Materials Required• Audio source (CD Player, Walkman, or Transistor radio)• Large magnifying glass of focal length around 15cm *(fairly standard)• Flashlight• Laser pointer (or other laser device)• Collection of remote control devices from several different manufacturers• Clear plastic bag• Colored plastic bag• Tissue• Piece of paper• Piece of cardboard• Phillips screw driver

*see section 1.5 for details

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5.2 Student Activity Sheet—Listening to Light 81

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5.2 PART I — THE PHOTOCELLName__________________________________ Date___________ Period__________

In this activity we use a detector (a photocell connected to a speaker) to “listen” to light. Ourdetector is capable of detecting light in both the visible and infrared parts of the electromagneticspectrum.

Experiment Results

A. Describe what you hear from the speakerwhen your teacher shines a flashlight onthe photocell detector.

B. Do you hear any difference as your teachermoves the flashlight closer and fartheraway from the photocell to make the lightdimmer and brighter?

C. Describe what you hear when your teachermoves her or his hand back and forthbetween the flashlight and the photocell.

D. Does the sound from the photocell detector result from the brightness of the light or fromchanges in the brightness of the light? What is your evidence?

Prediction Observation

E. What will you hear whenthe photocell detector isexposed to light from thefluorescent overheadlights?

F. Is the light from a fluorescent bulb constant or does it change? Explain how you know.

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PART II — YOUR OWN MUSIC STATION

Now we will explore how information can be carried by both visible and infrared light.

Experiment Results

A. What happens when a laser beam shineson the photocell from across the room.

Imagine that you had a similar apparatusset up between your house and yourfriend’s house at the end of the block. Howcould you use this arrangement to sendinformation to your friend?

B. Describe what you hear when your teacherhits the photocell with the signal from aremote control.

Is this information being transmitted usingvisible or invisible light?The small device in the end of the remotecontrol that produces the infrared light iscalled an infrared diode. Is the“brightness” of the light from the infrareddiode constant or does it flicker? How doyou know?

When the brightness of an infrared diode is changed using the output from an audio source, suchas a CD player, the information in the music can be transmitted to a photocell and on to aspeaker.

C. Explain carefully, possibly with a picture, why the volume of the music goes down as theinfrared diode is moved farther away from the photocell.

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PART III — WHAT GETS THROUGH?

In this part we will we look at how infrared light is similar to visible light and how it is different.

Experiment Result

A. What happens when the transmitter(infrared diode) is placed about 1 meterfrom the receiver?

B. What happens when your teacher uses amagnifying glass to focus the IR lightfrom the transmitter?

C. Predict (circle) what will happen to the volume of the music when each of the objects listedis placed between the diode and the photocell. Then record your observations after yourteacher performs each experiment.

1. A hand:Prediction: stay the same get slightly quieter get a lot quieter become silentObservation: stay the same get slightly quieter get a lot quieter become silent

2. A clear plastic bag:Prediction: stay the same get slightly quieter get a lot quieter become silentObservation: stay the same get slightly quieter get a lot quieter become silent

3. A colored plastic bag:Prediction: stay the same get slightly quieter get a lot quieter become silentObservation: stay the same get slightly quieter get a lot quieter become silent

4. A tissue:Prediction: stay the same get slightly quieter get a lot quieter become silentObservation: stay the same get slightly quieter get a lot quieter become silent

5. A piece of paper:Prediction: stay the same get slightly quieter get a lot quieter become silentObservation: stay the same get slightly quieter get a lot quieter become silent

6. A piece of cardboard:Prediction: stay the same get slightly quieter get a lot quieter become silentObservation: stay the same get slightly quieter get a lot quieter become silent

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D. When your teacher performed each of the experiments, were any of the results surprising?Which ones?

E. Space is not as empty as we often think it is. Much of space is filled with gas and dust thatastronomers call the interstellar medium. This dust can block visible light from distantobjects making them impossible to see. Why did the development of infrared telescopesallow astronomers to “see” these previously unseen objects?

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5.3 PART I — THE PHOTOCELLName__________________________________ Date___________ Period__________

In this activity we use a detector (a photocell connected to a speaker) to “listen” to light. Ourdetector is capable of detecting light in both the visible and infrared parts of the electromagneticspectrum.

Experiment Results

A. Describe what you hear from the speakerwhen your teacher shines a flashlight onthe photocell detector.

Initial “pop” when light from the flashlighthits, then nothing (but static) while the light

continues to shine on the photocell.

B. Do you hear any difference as your teachermoves the flashlight closer and fartheraway from the photocell to make the lightdimmer and brighter?

No. There may be an increase in static as theflashlight is moved farther away.

C. Describe what you hear when your teachermoves her or his hand back and forthbetween the flashlight and the photocell.

Popping sound every time the light hits thephotocell.

D. Does the sound from the photocell detector result from the brightness of the light or fromchanges in the brightness of the light? What is your evidence?

The photocell detects changes in the brightness of the light. There was no sound when theflashlight was close to the photocell (bright), and still no sound when it was farther away(dim). So the brightness of the light can’t be what’s detected. When the light is interrupted orchanged rapidly (when the hand was moved back and forth between the flashlight and thephotocell), there was a popping sound. This change in brightness was what was detected bythe photocell and sent to the speaker.

Prediction ObservationE. What you will hear when

the photocell detector isexposed to light from thefluorescent overheadlights?

Answers will vary.Fluorescent lights cause aconstant hum at a specific

pitch.

F. Is the light from a fluorescent bulb constant or does it change? Explain how you know.

The light from a fluorescent bulb changes. Because the photocell only detects light that’schanging, the constant hum produced in the speaker by the photocell proves the light isconstantly changing.

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PART II — YOUR OWN MUSIC STATION

Now we will explore how information can be carried by both visible and infrared light.

Experiment Results

A. What happens when a laser beam shineson the photocell from across the room.

Not much. There may be an initial pop whenthe laser first hits the photocell, but then

nothing (but static) while the laser continuesto shine on the photocell.

Imagine that you had a similar apparatusset up between your house and yourfriend’s house at the end of the block.How could you use this arrangement tosend information to your friend?

You would have to have some way to interruptor rapidly change the brightness of the laser

beam, so that it flashed in something likeMorse Code.

B. Describe what you hear when your teacherhits the photocell with the signal from aremote control.

Sound like a blaster gun from a science fictionmovie; the sound continues to fluctuaterapidly as long as the button is pressed.

Is this information being transmitted usingvisible or invisible light? Infrared light.

The small device in the end of the remotecontrol that produces the infrared light iscalled an infrared diode. Is the“brightness” of the light from the infrareddiode constant or does it flicker? How doyou know?

The brightness of light from the infrared diodemust flicker because sound was heard fromthe speaker and that only happens when the

brightness of light that strikes the photocell ischanging.

When the brightness of an infrared diode is changed using the output from an audio source, suchas a CD player, the information (music) can be transmitted to a photocell and on to a speaker.

C. Explain carefully, possibly with a picture, why the volume of the music goes down as theinfrared diode is moved farther away from the photocell.

As the infrared diode is moved farther away, the light it emits gets spread out over a largerarea and, therefore, appears dimmer. Thus the changes in the light that the photocell detectsare smaller and the signal sent to the speaker is smaller and the volume goes down.

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PART III — WHAT GETS THROUGH?

In this part we will we look at how infrared light is similar to visible light and how it is different.

Experiment Result

A. What happens when the transmitter(infrared diode) is placed about 1 meterfrom the receiver?

The music should not be able to be heardbecause the signal from the distant infrared

diode is too weak to be detected by thephotocell.

B. What happens when your teacher uses amagnifying glass to focus the IR lightfrom the transmitter?

You can hear the music again, although it maybe faint. The magnifying glass focused theinfrared light, just like it does visible light,increasing the brightness of the infrared

signal enough to be detected again.

C. Predict (circle) what will happen to the volume of the music when each of the objects listedis placed between the diode and the photocell. Then record your observations after yourteacher performs each experiment.

1. A hand:Prediction: stay the same get slightly quieter get a lot quieter become silentObservation: stay the same get slightly quieter get a lot quieter become silent

2. A clear plastic bag:Prediction: stay the same get slightly quieter get a lot quieter become silentObservation: stay the same get slightly quieter get a lot quieter become silent

3. A colored plastic bag:Prediction: stay the same get slightly quieter get a lot quieter become silentObservation: stay the same get slightly quieter get a lot quieter become silent

4. A tissue:Prediction: stay the same get slightly quieter get a lot quieter become silentObservation: stay the same get slightly quieter get a lot quieter become silent

5. A piece of paper:Prediction: stay the same get slightly quieter get a lot quieter become silentObservation: stay the same get slightly quieter get a lot quieter become silent

Which result you get depends on the thickness of the paper.6. A piece of cardboard:

Prediction: stay the same get slightly quieter get a lot quieter become silentObservation: stay the same get slightly quieter get a lot quieter become silent

Which result you get depends on the thickness of the cardboard.

Page 18: Listening to Light — Teacher Notes · Listening to Light — Teacher Notes ... When visible or infrared light strikes the solar cell, ... (next to the plastic piece

5.3 Teacher Answer Key—Listening to Light

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D. When your teacher performed each of the experiments, were any of the results surprising?Which ones?

Answers will vary, but students may express surprise that objects that block visible light letsome or all of the infrared light pass through them, e.g., the colored plastic bag, the paper,and the cardboard.

E. Space is not as empty as we often think it is. Much of space is filled with gas and dustastronomers call the interstellar medium. This dust can block visible light from distantobjects making them impossible to see. Why did the development of infrared telescopesallow astronomers to “see” these previously unseen objects?

Like the colored plastic bag, the paper, or the cardboard, interstellar dust blocks visible lightbut lets infrared light pass through it. Any objects located behind the dust and emittingvisible light cannot be seen. Astronomers using telescopes that can detect infrared light can“see” through the dust to the objects emitting the infrared light, just as the photocell could“see” the brightness changes from the infrared diode (produced by the music) through thecolored plastic bag, paper and cardboard.